1-Alkyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids as highly safe electrolyte for Li/LiFePO4 battery

Several 1-alkyl-2,3-dimethylimidazolium bis(trifluoromethanesulfonyl)imide ionic liquids (alkyl-DMimTFSI) were prepared by changing carbon chain lengths and configuration of the alkyl group, and their electrochemical properties and compatibility with Li/LiFePO₄ battery electrodes were investigated in detail. Experiments indicated the type of ionic liquid has a wide electrochemical window (−0.16 to 5.2 V vs. Li⁺/Li) and are theoretically feasible as an electrolyte for batteries with metallic lithium as anode. Addition of vinylene carbonate (VC) improves the compatibility of alkyl-DMimTFSI-based electrolytes towards lithium anode and LiFePO₄ cathode, and enhanced the formation of solid electrolyte interface to protect lithium anodes from corrosion. The electrochemical properties of the ionic liquids obviously depend on carbon chain length and configuration of the alkyl, including ionic conductivity, viscosity, and charge/discharge capacity etc. Among five alkyl-DMimTFSI-LiTFSI-VC electrolytes, Li/LiFePO₄ battery with the electrolyte-based on amyl-DMimTFSI shows best charge/discharge capacity and reversibility due to relatively high conductivity and low viscosity, its initial discharge capacity is about 152.6 mAh g⁻¹, which the value is near to theoretical specific capacity (170 mAh g⁻¹). Although the battery with electrolyte-based isooctyl-DMimTFSI has lowest initial discharge capacity (8.1 mAh g⁻¹) due to relatively poor conductivity and high viscosity, the value will be dramatically added to 129.6 mAh g⁻¹ when 10% propylene carbonate was introduced into the ternary electrolyte as diluent. These results clearly indicates this type of ionic liquids have fine application prospect for lithium batteries as highly safety electrolytes in the future.     

A novel intrinsically porous separator for self-standing lithium-ion batteries

γ-LiAlO₂, Al₂O₃ and MgO were used as fillers in a PVdF-HFP polymer matrix to form self-standing, intrinsically porous separators for lithium-ion batteries. These separators can be hot-laminated onto the electrodes without losing their ability to adsorb liquid electrolyte. The electrochemical stability of the separators was tested by constructing half-cells with the configuration: Li/fibre-glass/filler-based separator/electrode. MgO-based separators were found to work well with both positive and negative electrodes. An ionic conductivity of about 4×10⁻⁴ S cm⁻¹ was calculated for the MgO-based separator containing 40% 1 M solution of LiPF₆ in an EC/DMC 1:1 solvent. Self-standing, lithium-ion cells were constructed using the MgO-based separator and the resulting battery performance evaluated in terms of cyclability, power and energy density.     

A highly conductive organic-inorganic hybrid electrolyte based on co-condensation of di-ureasil and ethylene glycol-containing alkoxysilane

Organic-inorganic hybrid electrolytes based on di-ureasil backbone structures by reacting poly(propylene glycol)-block-poly(ethylene glycol)-block-poly(propylene glycol) bis(2-aminopropyl ether) (ED2000) with 3-(triethoxysilyl)propyl isocyanate (ICPTES), followed by co-condensation with methoxy(polyethylenoxy)propyl trimethoxysilane (MPEOP) in the presence of LiClO₄ were prepared and characterized by a variety of techniques. The hybrid electrolytes showed good resistance to crystallization and excellent conductivity for use in lithium-ion batteries, as determined by differential scanning calorimetry (DSC) and impedance measurements, respectively. The temperature dependence of the ionic conductivity exhibited a VTF (Vogel-Tamman-Fulcher)-like behavior for all the compositions studied and a maximum ionic conductivity value of 6.9 × 10⁻⁵ S cm⁻¹, a relatively high value for solid polymer electrolytes, was achieved at 30 °C for the hybrid electrolyte with a [O]/[Li] ratio of 16. A microscopic view of the dynamic behavior of the polymer chains (¹³C) and the ionic species (⁷Li) was provided by the ¹H and ⁷Li line widths measured from 2D ¹H-¹³C WISE (Wideline Separation) and variable temperature ⁷Li static NMR, respectively, to elucidate the influence of the mobility of the polymer chains and the charge carriers on the observed ionic conductivity. The present salt-free hybrid electrolyte after plasticization with 1 M LiClO₄ in EC/PC solution exhibited a swelling ratio of 275% and reached an ionic conductivity value up to 8.3 × 10⁻³ S cm⁻¹ at 30 °C, which make it a good candidate for the further development of advanced rechargeable lithium-ion batteries.     

Preparation and electrochemical behaviors of polymeric composite electrolytes containing mesoporous silicate fillers

In the present study, poly(ethylene oxide) (PEO)-based polymeric composite electrolytes (PCEs) had been prepared by using a different content of mesoporous silicate MCM-41, in order to examine the filler addition effect on the microstructural and electrochemical properties. The interactions between MCM-41 and PEO matrix were studied by XRD, DSC, and SEM techniques. The electrochemical properties of the PCEs, such as ionic conductivity, its temperature dependence, and lithium transference number were investigated. MCM-41 could maintain the pore structure effectively, resulting in nanocomposites that were homogeneously complexed with the PEO chains. The PCEs with 8 wt.% MCM-41 showed the smallest crystallinity, 30.4%. Accordingly, those PCEs showed the highest ion conductivity, 1.2 × 10⁻⁴ S/cm, a two-order-of-magnitude higher value than that of the pristine PEO-LiClO₄. This might have reflected decreased crystallinity and improved ion transport. Furthermore, those PCEs showed an increased Li ion transference number of ∼0.5. In conclusion, the filler addition could enhance the ionic conductivity and increase the Li ion transference number at the same time.     

Separator grafted with siloxane by electron beam irradiation for lithium secondary batteries

Polyethylene (PE) separator grafted with 2,4,6,8-tetramethyl-2,4,6,8-tetravinylcyclotetrasiloxane (siloxane) was newly prepared by electron beam irradiation. The degree of grafting and morphology of the grafted separators were characterized by FT-IR and scanning electron microscopy (SEM). The polymer electrolytes based on the grafted separators were prepared by immersing the separators in the electrolyte containing 1 M LiPF₆ in EC/DMC (1:1 by volume). The ionic conductivity of the grafted separators was changed with the degree of grafting and showed the highest value of 7 × 10⁻⁴ S cm⁻¹ at the degree of grafting of 6%. The electrochemical stability limit of the grafted separator with the degree of grafting of 6% was increased to 5.2 V. The Li ion cell using the grafted separator also showed an improved performance, suggesting that the grafted separator is a good candidate for the separator of lithium batteries at high voltage operation.     

Preparation of a microporous polymer electrolyte based on poly(vinyl chloride)/poly(acrylonitrile-butyl acrylate) blend for Li-ion batteries

Poly(acrylonitrile-co-butyl acrylate) (P(AN-co-BuA))/poly(vinyl chloride) (PVC) blend-based gel polymer electrolyte (BGPE) was prepared for lithium-ion batteries. The P(AN-co-BuA)/PVC BGPE consists of an electrolyte-rich phase, which is mainly composed of P(AN-co-BuA) and liquid electrolyte, acting as a conducting channel and a PVC-rich phase that provides mechanical strength. The dual phase was just simply developed by the difference of miscibility properties in solvent, PC, between P(AN-co-BuA) and PVC. The mechanical strength of this new blend electrolyte was found to be much higher, with a fracture stress as high as 29 MPa in dry membrane and 21 MPa in gel state, than that of a previously reported P(AN-co-BuA)-based gel polymer electrolyte. The blended gel polymer electrolyte showed ionic conductivity of higher than 1.5 × 10⁻³ S cm⁻¹ and electrochemical stability up to at least 4.8 V. The results showed that the as-prepared gel polymer electrolytes were promising materials for lithium-ion batteries.     

Preparation of poly(acrylonitrile-butyl acrylate) gel electrolyte for lithium-ion batteries

Poly(acrylonitrile-butyl acrylate) gel polymer electrolyte was prepared for lithium ion batteries. The preparation started with synthesis of poly(acrylonitrile-butyl acrylate) by radical emulsion polymerization, followed by phase inversion to produce microporous membrane. Then, the microporous gel polymer electrolytes (MGPEs) was prepared with the microporous membrane and LiPF₆ in ethylene carbonate/diethyl carbonate. The dry microporous membrane showed a fracture strength as high as 18.98 MPa. As-prepared gel polymer electrolytes presented ionic conductivity in excess of 3.0 × 10⁻³ S cm⁻¹ at ambient temperature and a decomposition voltage over 6.6 V. The results showed that the as-prepared gel polymer electrolytes were promising materials for Li-ion batteries.     

Formation and electrochemical performance of copper/carbon composite nanofibers

Copper-loaded carbon nanofibers are fabricated by thermally treating electrospun Cu(CH₃COO)₂/polyacrylonitrile nanofibers and utilized as an energy-storage material for rechargeable lithium-ion batteries. These composite nanofibers deliver more than 400 mA g⁻¹ reversible capacities at 50 and 100 mA g⁻¹ current densities and also maintain clear fibrous morphology and good structural integrity after 50 charge/discharge cycles. The relatively high capacity and good cycling performance of these composite nanofibers, stemmed from the integrated combination of metallic copper and disordered carbon as well as their unique textures and surface properties, make them a promising electrode candidate for next-generation lithium-ion batteries.     

Ternary polymer electrolytes with 1-methylimidazole based ionic liquids and aprotic solvents

New polymer gel electrolytes containing ionic liquids were developed for modern chemical power sources—supercapacitors and lithium-ion batteries. Ternary systems polymer-ionic liquid-aprotic solvent as well as materials containing also lithium salts (LiClO₄ or LiPF₆) were prepared by direct, thermally initiated polymerisation. Poly(2-ethoxyethyl methacrylate) PEOEMA was combined with various ionic liquids based on 1-methylimidazole. Only 1-butyl-3-methylimidazolium hexafluorophosphate BMIPF₆ formed a homogenous and slightly translucent polymer electrolyte, where aprotic solvents—propylene carbonate and ethylene carbonates were used as plasticisers. Materials were studied using the electrochemical and thermogravimetric methods and exhibit high ionic conductivity up to 0.94 mS cm⁻¹ at 25 °C together with high electrochemical stability: the accessible potential window on the glassy carbon was found ca. 4.3 V. Prepared non-volatile materials are long-term and thermally stable up to 150 °C.     

Ionic conductivity of chitosan membranes

Chitosan membranes with various degrees of deacetylation and different molecular weights (MW) were prepared by film casting with aqueous solutions of chitosan and acetic acid. Ultraviolet (UV) spectrometry and infrared (IR) spectrometry were used to determine the degree of deacetylation (DDA) of chitosan. The viscosity-average MW of chitosan was measured in an aqueous solvent system of 0.25 M CH₃COOH/0.25 M CH₃COONa. The intrinsic ionic conductivities of the hydrated chitosan membranes were investigated using impedance spectroscopy. It was found that the intrinsic ionic conductivity was as high as 10⁻⁴ S cm⁻¹ after hydration for 1 h. The tensile strength and breaking elongation of the membranes were evaluated according to standard ASTM methods. The crystallinity and swelling ratio of the membranes were examined. A tentative mechanism for the ionic conductivity of chitosan membranes is also suggested.     

Nonaqueous and template-free synthesis of Sb doped SnO2 microspheres and their application to lithium-ion battery anode

Antimony doped SnO₂ (ATO) microspheres composed of ATO nanoparticles were prepared by using a hydrothermal process in a nonaqueous and template-free solution from the inorganic precursors (SnCl₄ and Sb(OC₂H₅)₃). The physical properties of the as-synthesized samples were investigated by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), N₂ adsorption-desorption isotherms, and X-ray photoelectron spectrum (XPS). The resulting particles were highly crystalline ATO microspheres in the diameter range of 3-10 μm and with many pores. The as-prepared samples were used as negative materials for lithium-ion battery, whose charge-discharge properties, cyclic voltammetry, and cycle performance were examined. The results showed that a high initial discharge capacity of 1981 mAh g⁻¹ and a charge capacity of 957 mAh g⁻¹ in a potential range of 0.005-3.0 V was achieved, which suggests that tin oxide-based materials work as high capacity anodes for lithium-ion rechargeable batteries. The cycle performance is improved because the conducting ATO nanoparticles can also perform as a better matrix for lithium-ion battery anode.     

Effect of surface modified porous inorganic-organic hybrid polyphosphazene nanotubes on the properties of polyethylene oxide based solid polymer electrolytes

2-(2-methyloxyethoxy)ethanol modified poly (cyclotriphosphazene-co-4,4′-sufonyldiphenol) (PZS) nanotubes were synthesized and solid composite polymer electrolytes based on the surface modified polyphosphazene nanotubes added to PEO/LiClO₄ model system were prepared. Differential Scanning Calorimetry (DSC) and Scanning Electron Microscopy (SEM) were used to investigate the characteristics of the composite polymer electrolytes (CPE). The ionic conductivity, lithium ion transference number and electrochemical stability window can be enhanced after the addition of surface modified PZS nanotubes. The electrochemical investigation shows that the solid composite polymer electrolytes incorporated with PZS nanotubes have higher ionic conductivity and lithium ion transference number than the filler SiO₂. Maximum ionic conductivity values of 4.95 × 10⁻⁵ S cm⁻¹ at ambient temperature and 1.64 × 10⁻³ S cm⁻¹ at 80 °C with 10 wt % content of surface modified PZS nanotubes were obtained and the lithium ion transference number was 0.41. The good chemical properties of the solid state composite polymer electrolytes suggested that the inorganic-organic hybrid polyphosphazene nanotubes had a promising use as fillers in solid composite polymer electrolytes and the PEO₁₀-LiClO₄-PZS nanotubes solid composite polymer electrolyte can be used as a candidate material for lithium polymer batteries.     

Preparation and electrochemical characterization of the alkaline polymer gel electrolyte polymerized from acrylic acid and KOH solution

An alkaline polymer gel electrolyte (PGE) film was prepared by solution polymerization of acrylate-KOH-H₂O at room temperature, and the preparation conditions were optimized in view of the mechanical properties and ionic conductivity of the film. The PGE film with the optimized composition of 0.02% K₂S₂O₈, 16.75% acrylic acid and 83.23 wt.% 4 mol l⁻¹ KOH solution is transparent, rubber-like and dimensionally stable with improved mechanical properties as compared with gelled electrolyte. The specific conductivity of the film is 0.288 s cm⁻¹ at room temperature and the conductivity values follow the Arrhenius equation with the activation energy of ∼10 kJ mol⁻¹. These data suggest that the ionic conduction proceeds in the same mechanism as in aqueous alkaline solution. Experimental results from the laboratory Zn/Air, Zn/MnO₂ and Ni/Cd cells using the PGE film as electrolyte demonstrate that the PGE film has almost the same chemical and electrochemical stability as aqueous alkaline solution, and shows good performance characteristics for application of alkaline primary and secondary battery systems.     

Highly ion conductive flexible films composed of network polymers based on polymerizable ionic liquids

Ionic liquid-type polymer brushes having different hydrocarbon (HC) chain lengths between polymerizable group and imidazolium ring were synthesized. When the carbon number of HC chain was 6, the ionic liquid-type polymer brush exhibited the highest ionic conductivity of 1.37×10⁻⁴ S cm⁻¹ at 30 °C, reflecting low T_g of −60 °C. Moreover, for the first time, we succeeded in obtaining transparent and flexible films without considerable decrease in the ionic conductivity as compared with that of corresponding monomers by using suitable cross-linkers. The most ion conductive (1.1×10⁻⁴ S cm⁻¹ at 30 °C) film was obtained when tetra(ethylene glycol)diacrylate was used 0.5 mol% to ionic liquid monomer as the cross-linker. This film is one of excellent conductive films among single-ion conductive materials.     

All solid-state polymer electrolytes prepared from a graft copolymer consisting of a polyimide main chain and poly(ethylene oxide) based side chains

We prepare an all solid-state, liquid-free, polymer electrolyte (ASPE) from a lithium salt and a graft copolymer consisting of a polyimide main chain and poly(ethylene glycol) methyl ether methacrylate side chains using atom transfer radical polymerization method. The ionic conductivity of ASPEs increases with increasing the side chain length. The ionic conductivity of the ASPE whose POEM content = 60 wt% shows 6.5 × 10⁻⁶ S/cm at 25 °C. The ASPEs having shorter average distance between side chains and/or shorter side chain length show higher mechanical strength. The tensile strength of the ASPEs is more than 10 MPa and about 20 times higher than that of the ASPEs in the previous study [Electrochim. Acta, 50 (1998) 3832]; hence, the ASPEs have sufficiently high mechanical strength for a polymer electrolyte of lithium secondary batteries.     

Effect of zwitterionic salt on the electrochemical properties of a solid polymer electrolyte with high temperature stability for lithium ion batteries

In this study, we prepare a kind of solid polymer electrolyte (SPE) based on N-ethyl-N′-methyl imidazolium tetrafluoroborate (EMIBF₄), LiBF₄ and poly(vinylidene difluoride-co-hexafluoropropylene) [P(VdF-HFP)] copolymer. The resultant SPE displays high thermal stability above 300 °C and high room temperature ionic conductivity near to 10⁻³ S cm⁻¹. Its electrochemical properties are improved with incorporation of a zwitterionic salt 1-(1-methyl-3-imidazolium)propane-3-sulfonate (MIm3S). When the SPE contains 1.0 wt% of the MIm3S, it has a high ionic conductivity of 1.57 × 10⁻³ S cm⁻¹ at room temperature, the maximum lithium ions transference number of 0.36 and the minimum apparent activation energy for ions transportation of 30.9 kJ mol⁻¹. The charge-discharge performance of a Li₄Ti₅O₁₂/SPE/LiCoO₂ cell indicates the potential application of the as-prepared SPE in lithium ion batteries.     

Electrochemical behavior of Ge and GeX2 (X = O, S) glasses: Improved reversibility of the reaction of Li with Ge in a sulfide medium

Sulfide glasses have been considered as new anode materials for lithium-ion batteries because their high ionic conductivity (approximately ≥10⁻⁴ S/cm) (more than one order of magnitude higher than oxide glasses (approximately ≤10⁻⁶ S/cm)) was expected to accelerate Li⁺ ion insertion into and extraction from anode materials during charge and discharge reactions. This intrinsic property can yield the reversible lithium-alloying reaction by minimizing the aggregation of lithium-alloy phases leading to the improvement of cycling behavior. To examine sulfide glasses as new anode materials, GeS₂ glass was chosen for study in this work due to its stability in air-atmospheres. The electrochemical properties of the GeS₂ glass were compared with those of the Ge metal and GeO₂ glass. The initial insertion of lithium into the GeX₂ (X = O, S) glasses leads to the formation of Li₂X (X = O, S) phases associated with the irreversible capacity on the first cycle. The improved reversibility of the reaction of lithium with Ge was observed in the Li₂S medium rather than Li₂O one, which leads to the improvement of cycle performance in the GeS₂ glass anode.     

Flower-like ZnO-NiO-C films with high reversible capacity and rate capability for lithium-ion batteries

Flower-like ZnO-NiO-C films with high reversible capacity and rate capability for lithium-ion batteries were fabricated through simple solution-immersion steps and subsequent heat treatment at moderate temperature. At a rate of 0.5 C, reversible capacity greater than 485 mAh g⁻¹ could be retained at the 50th cycle for ZnO-NiO-C films. More importantly, the films delivered reversible capacities of 380, 300, 230, and 180 mAh g⁻¹ at 1, 2, 3 and 4 C rates, respectively. The superior electrochemical properties of the ZnO-NiO-C films resulted from the advantages of flower-like architecture as well as the catalytic and conductive effects of the Ni phase produced in the first discharge process. Owing to easy fabrication and excellent electrochemical performance, these ZnO-NiO-C films will be promising anodes for lithium-ion batteries. The results of this study also offer possibilities of improving the lithium storage capacity of transition metal oxides by controlling both architecture and composition.     

Multi-functional Al2O3-coated separators for high-performance lithium-ion batteries: Critical effects of particle shape

The technology of coating polyolefin-type separators with ceramics is gradually developing as an effective method to improve the safety of lithium-ion batteries (LIBs). However, the powder properties of ceramics can adversely affect the surface structure and ionic conductivity of separators; therefore, a new approach is required regarding the powder properties that affect the performance of the separator. Herein, the effect of the Al₂O₃ particle shape on the physical properties of Al₂O₃-coated separators and the performance of LIBs is investigated. In the separator coated with angular-shaped Al₂O₃ particles (Al₂O₃-A), the pores in the coating layer are uniformly distributed, improving physical properties such as porosity and wettability. The thermal shrinkage of separator is <10% when exposed to 150 °C for 1 h, considerably smaller than that of the commercial polyethylene separator (approximately 83%) under the same conditions. Moreover, the Al₂O₃-A-coated separator shows the highest ionic conductivity (0.531 mS cm⁻¹), and the LiNi_0.8Mn_0.1Co_0.1O₂/Al₂O₃-A-coated separator/Li battery displays improved stability than using the polyethylene separator under a current density of 5C. This proposes approach to improve the separator's performance through the shape control of ceramic particles paves the way for separators to contribute to the high-temperature safety and long cycle life of batteries.